The present invention relates to monolithic integrated circuits and, more particularly, to monolithic integrated circuits which initially have pairs of regions isolated from one another which are to be selectively interconnected permanently.
A number of techniques have been resorted to to permit selective, permanent interconnections of circuit portions in monolithic integrated circuits after the fabrication process for such integrated circuits has been completed or nearly completed. A commonly encountered situation in which the option to permanently interconnect portions of a circuit, or to form a plurality of such interconnections for a plurality of circuits, is the selective interconnection of portions of read-only memories to thereby encode them. Another situation is to increase operable chip yield, and so reduce waste in the fabricating of monolithic integrated circuits through use of redundant circuit portions on the chips which are can be selectively interconnected in place of corresponding failed circuit portions. Yet another use is in the "trimming" of resistance values of integrated circuit resistors to leave them with a desired resistance value.
Among the techniques used to provide interconnection capabilities for such purposes is the use of doped polycrystalline silicon links or conductive metal links between two regions which can be selectively removed through irradiating them with a laser beam having sufficient energy to vaporize portions of such links. However, the vaporizing of these links by the laser can lead to damage of the materials underlying these links. Further, if the links are irradiated before a passivation or other protective layer is provided over them, this irradiation to remove the links can lead to the ejecting of link materials from the link to the surrounding regions which allows the possibility of adverse effects in those regions by such debris. On the other hand, irradiating the links after they have been covered by a protective layer can lead to large vapor pressures under the passivation layer which may result in a cracked or otherwise damaged passivation layer.
Other techniques have required special structures or additional structural features differing from those found in typical integrated circuit structures requiring extra fabrication steps and so increased costs. Some of the structures used as links require substantial currents to flow to sufficiently damage these links so as to cause them to melt or vaporize, again leading to debris and substantial heating which can adversely affect the monolithic integrated circuit.
Thus, there is desire to find a way of providing an option for selectively permanently interconnecting regions in a monolithic integrated circuit which can be provided through the same fabrication steps used in fabricating the remaining portions of the integrated circuit. Such an interconnection option must provide a reliable interconnection without collateral damage to other structural features.